The present invention relates to a serial data communications system and a system control method and more particularly to a technique that can be effectively applied to such systems as video tape recorders (simply referred to as VTRs), video disc players (VDPs), compact disc players, digital audio tape recorders (DATs) and television sets (TVs), which have various servo control ICs (integrated circuits) and many signal processing ICs and in which multiple operations are initiated simultaneously by means of mode specification.
Known communications systems that perform the serial data communication by two-wire circuit include Japan Patent Publication No.65178/1988 (equivalent U.S. Pat. No. 4,689,740, European Patent No.51332, and South Korea Patent No. 8801017). This system employs a two-line circuit consisting of a clock line for supplying clock signals and a data line for supplying data. In a normal data transfer, the data signal transition is allowed only during the low state of the clock, except that a "start signal" and an "end signal" are made to change their states during the high-state period of the clock.
The inventors have found the following.
When a plurality of receiving stations connected to the same bus line are to be controlled individually by using addresses, what always matters is a simultaneity of controls. This is an important issue with VTR and VDP where the mechanical control and the signal processing are closely related with each other.
When, in VTR for example, the mode is to be switched from a playback mode to a record mode, the operation mode of the video signal processing IC is switched from the playback mode to the record mode. At this time, to prevent the switching noise from being recorded on the magnetic tape, it is necessary to mute the video signal processing IC. At the same time, a playback preamplifier is stopped or deenergized, the record amplifier is energized, and a head switch is operated accordingly. The servo system control IC must change the phase/speed control of capstan and drum from the playback to the record mode. The audio signal processing IC is also switched in the operation mode from the playback to the record mode. To prevent abnormal noise from occurring during the transient state, the audio signal processing IC must be muted.
These controls are supposed to be initiated simultaneously, rather than being performed sequentially as in the conventional serial data transfer method. In the conventional method, as the data transfer speed is increased, the simultaneity of controls tends to improve. However, this calls for a device capable of high-speed operation, which in turn pushes up the overall cost of the data processing system. An increased speed of the data transfer has another drawback of easily producing high frequency interferences.